1. Field of the Invention
This invention relates to a fluorophosphate optical glass having optical constants, i.e. a refractive index (nd) of 1.42 to 1.47 and Abbe number (.nu.d) of 90 to 97. The low refractive index and low dispersion optical glass of the present invention has similar optical properties to those of crystalline calcium fluoride and is thus useful as a glass material serving for calcium fluoride as a lens or optical part in an optical instrument.
2. Description of the Prior Art
Crystalline calcium fluoride, which has hitherto been used as a low refractive index and low dispersion optical material for a camera lens, meets with various problems because of being crystalline, for example, that the chemical resistance and workability are not good and the productivity is low to disadvantage mass production of it. In order to solve these problems, an optical glass having similar optical properties to crystalline calcium fluoride has thus been desired and some glasses have been proposed which consist of low refractive index and low dispersion fluorophosphates.
For the purpose of realizing such optical properties, in particular, a low dispersion, fluoride ion must be introduced as an anion serving for oxide ion in a glass and when the Abbe number is above 90, the proportion of the fluoride ions is considerably increased as compared with the oxide ions. Such a glass tends to be crystallized more than ordinary optical glasses and is so unstable that the production thereof on a commercial scale, as having uniform properties, is very difficult.
In view of the situation, several fluorophosphate glasses have thus been reported as disclosed in Japanese Patent Publication Nos. 34768/1979 and 14378/1983 and Japanese Patent Application OPI Nos. 43112/1975, 81042/1985 and 210545/1985. However, these glasses have such problems that large amounts of volatile components contained therein evaporate during melting, thus resulting in a fluctuation of the refractive index, and the production of those containing relatively expensive and special components, on a commercial scale, meets with an economical problem. Furthermore, the above proposed glasses are not satisfactorily in the effect for crystallization. Therefore, a great deal of efforts are required so as to obtain a sufficiently stable and uniform glass.